Earlier, Trump suggested the demilitarised zone at Panmunjom between North and South Korea would be a possible venue for the breakthrough meeting.

Singapore emerged as the likely host of the summit after Trump yielded to the concerns of his aides and backed off his desire to hold the meeting at the inter-Korean demilitarised zone.

Angry rhetoric

Plans for a summit between Trump and Kim follow months of angry exchanges between the two over Pyongyang's testing of nuclear weapons and long-range missiles, including some theoretically capable of reaching the US mainland.

But a surprising about-turn by Kim in recent months raised hopes of a turning point.

Trump thanked North Korean leader for releasing the Americans and said he believes Kim wants to reach an agreement on denuclearisation at their upcoming summit. "I really think he wants to do something," the president said.

US Vice President Mike Pence said on Thursday on NBC News, "In this moment, the regime in North Korea has been dealing, as far as we can see, in good faith."

Seoul and Pyongyang have been technically at war since the 1950s because the conflict ended in a truce, not a peace treaty.

But South Korean President Moon Jae-in and Kim agreed at a landmark summit last week to work towards a permanent treaty to replace the 65-year-old armistice.

Preparations for the Trump-Kim meeting gathered further momentum since the Korean summit.

South Korea's government said it had high hopes.

"We welcome the North Korea-US summit to be held in Singapore on June 12. We hope the denuclearisation of the Korean Peninsula, as well as permanent peace on the peninsula, will successfully come about through this summit."